The present invention relates to a telephone exchange apparatus for performing exchange processing and a tone sending method in the telephone exchange apparatus.
As a conventional telephone exchange apparatus, for example, an apparatus constituted by a tone generator for generating first tone signals, and a memory time switch for generating second tone signals by sequentially exchanging the first tone signals generated by the tone generator, and sending the second tone signals to corresponding output time slots is known.
FIG. 6 is a block diagram showing an example of such a conventional telephone exchange apparatus.
In FIG. 6, a tone generator 500 for generating first tone signals is connected to a memory time switch 502 through a PCM highway 501.
A line interface 504 accommodates an extension telephone set 508 through an extension line 507. The line interface 504 is connected to a telephone network 510 through a line 509, and is further connected to the memory time switch 502 through a PCM highway 503.
A controller 506 controls the entire system, and is connected to the memory time switch 502 through a signal line 505.
An operation for generating second tone signals based on the first tone signals by switching the memory time switch 502 in the conventional telephone exchange apparatus with the above-mentioned arrangement will be described below.
The first tone signals generated by the tone generator 500 are continuously sent onto the highway 501. The controller 506 generates second tone signals based on the first tone signals sent from the highway 501 by switching a connection of the memory time switch 502. The generated second tone signals are sent onto a PCM highway 511 and are input to the memory time switch 502 again.
FIG. 7 is a waveform chart showing waveforms of conventional tone signals in the above-mentioned memory time switch 502.
The conventional controller 506 controls the memory time switch 502 through the signal line 505 so as to generate the second tone signals based on the first tone signals. Therefore, the switching time may often coincide with the peak level of the first tone signal. When there is a switching timing 1 which coincides with the peak level of the first tone signal, the second tone signal becomes a signal including a harmonic component at the switching timing. The second tone signal in this case corresponds to a waveform (511) shown in FIG. 7.
The controller 506 also performs control for sending the second tone signals generated in this manner to the line interface 504. The tone signals are sent to the line interface 504 by switching the memory time switch 502. Therefore, at this switching timing 2, a signal (503) including a harmonic component is also generated and sent, as indicated by a "sending tone signal" in FIG. 7.
As described above, in the prior art, the second tone signals are generated by switching the first tone signals at random timings. For this reason, the smooth first tone signal including only a frequency component in an audio band (0.3 to 3.4 kHz) is disconnected at random timings. For this reason, the amplitude of the second tone signal upon disconnection has a random value, and the second tone signal includes a harmonic component depending on the amplitude value at that time.
The second tone signals generated by the memory time switch 502 are randomly switched by returning them to the memory time switch 502 so as to generate the sending tone signals, and the sending tone signals are sent to corresponding time slots. For this reason, the second tone signals are further converted to signals including harmonic components, and the converted signals are sent.
Therefore, the prior art provides an apparatus having poor tone quality and sending unnecessary noise to an extension telephone set or to another system through, e.g., an outside line, and an operator feels discomfort.